1. Field of the Invention
The present invention relates to a technique for imaging regional cerebral blood flow (CBF) noninvasively using MRI with radio frequency arterial spin labeling and, more particularly, to a labeling technique which permits extension of this technique to a multi-slice examination.
2. Description of the Prior Art
The measurement of regional cerebral blood flow (CBF) is known to provide useful information both about cerebrovascular sufficiency and about regional brain metabolism because resting CBF and tissue metabolism are often strongly coupled. CBF imaging with Single Photon Computed Tomography (SPECT), Positron Emission Tomography (PET) and Xenon enhanced CT have been used to evaluate a multitude of cerebral disorders including stroke, dementia, epilepsy, trauma and neoplasms. PET imaging of CBF has also been an important tool for mapping task induced brain activity in normal and pathologic states. However, for most central nervous system disorders, Magnetic Resonance Imaging (MRI) provides the greatest sensitivity to structural abnormalities. A robust MRI based method for clinical imaging of CBF would allow both an anatomical and a functional assessment within the same examination. In addition to providing direct structure-function correlation, the higher spatial resolution of MRI as compared to nuclear medicine methods will lead to better image quality.
MRI of CBF can be performed either with intravascular contrast agents or by Arterial Spin Labeling (ASL). As known to those skilled in the art, instead of using an exogenous tracer, ASL electromagnetically labels water proton spins in the feeding arteries before they flow into the tissue. ASL is attractive since it can reduce the risk, complexity, and cost of a study. It is also more readily quantified and repeated than contrast agent methods.
Arterial Spin Labeling (ASL) techniques can be crudely divided into pulsed inversion techniques, such as EPISTAR, FAIR and other variants, and techniques which employ continuous arterial spin labeling (CASL). CASL produces more than twice the signal of pulsed techniques. Unfortunately, with both methods, multi-slice imaging is complicated by effects of the electromagnetic labeling on the image intensity, as well as the relatively long time required for blood to flow from the arteries into the thick slab of tissue to be imaged.
The ASL approach to MRI of CBF offers the potential for completely non-invasive, quantitative imaging of an important physiologic and diagnostic quantity; however, practical implementations of ASL in humans have typically suffered from systematic errors and artifacts which can only be corrected one slice at a time and which have thus limited its applicability. Recently, the use of fast imaging methods and an increased understanding of the factors affecting quantification of CBF have led to the acquisition of single-slice CBF images of good quality such that clinical and research applications can be explored. Unfortunately, most applications require greater slice coverage but the serial acquisition of many slices can be prohibitively time consuming. However, improved Radio Frequency (RF) pulse shapes and sequence design has made multi-slice pulsed ASL techniques feasible and multi-slice perfusion imaging more practical for diagnosis.
Accordingly, a method is desired which makes possible multi-slice imaging of CBF with good signal-to-noise ratio and arbitrary angles between imaging and labeling planes. The present invention is designed to meet this need in the art.